Field of the Invention
Embodiments of the present invention generally relate to a system and method for sensing the position of input objects, and the force or pressure applied by input objects to the surface of a touch sensing device.
Description of the Related Art
Input devices including touch sensor devices, also commonly called touchpads or touch sensor devices, are widely used in a variety of electronic systems. A touch sensor device typically includes a device sensing region in which the touch sensor device determines the presence, location and/or motion of one or more input objects, such as a finger. Touch sensor devices may be used to provide interfaces for an electronic system. For example, touch sensor devices are often used as input devices for larger computing systems, such as opaque touchpads integrated in, or peripheral to notebook or desktop computers. Touch sensor devices are also often used in smaller computing systems, such as touch screens integrated in cellular phones. Touch sensor devices are typically used in combination with other supporting components, such as display or input devices found in the electronic or computing system. Examples of some typical touch sensor device applications are components that are formed within or include a touch screen for a desktop computers, a touch screen for a laptop computer, netbook computers, tablets, web browsers, e-book readers, personal digital assistants (PDAs), smart phones and other similar electronic devices.
In some configurations, the touch sensor devices are coupled to these supporting components to provide a desired combined function or to provide a desirable complete device package. Many commercially available touch sensor devices rely on the measurement of an electrical property such as capacitance or resistance in order to determine the presence, location and/or motion of one or more input objects within the touch sensor device's active area. In order to simultaneously measure the position of multiple input objects, typical touch sensors employ an array of independent touch-sensing elements. The touch-sensing elements in the vicinity of any particular input object produce measurement data that can be used to determine the position of that input object, independently from the positions of other input objects that might be present elsewhere in the touch sensor's active area. Typically, a capacitive touch sensor device utilizes two overlapping arrays of sensor electrodes to detect the presence, location and/or motion of input objects. The touch-sensing elements are generally located in the areas where the electrodes overlap one another, and the electrodes are typically connected to controlling electronics with wires or conductive traces.
In most cases, capacitive sensing techniques are not effective for detecting the positions of input objects that have insulating properties, such as dielectric-containing or dielectric-coated objects (e.g., plastic styluses, rubber tipped pens, gloved hands, etc.). Therefore, to overcome this problem, device manufacturers have developed devices that can sense the physical touch of an input object on the surface of the touch sensing device's interface, such as resistive touch sensing devices and piezoelectric touch sensing devices. However, resistive touch sensing devices become unreliable over time, due to the mechanical stresses associated with repetitive movement, repetitive contact of the electrical contacting elements, and the large deflection often required to reliably distinguish between a touched and a non-touched state.
Some touch sensing devices have used piezoelectric materials to sense the presence and force or pressure from an input object. Such devices generally rely on the piezoelectric effect, which causes an electric charge to form in the piezoelectric material when a force is applied. However, the electric charge that is generated by the applied force is transient in nature due to the fact that piezoelectric materials are not perfect insulators and the charge moves or “bleeds away” over time. The transient nature of the generated charge will thus not allow the touch sensing device to detect the presence or non-presence of a stationary input object (e.g., finger) after a very short amount of time.
To construct a two-dimensional array of piezoelectric sensors, suitable for detecting both the position and the applied pressure of one or more input objects at the same time, each sensor in the array would typically need to have a separate sense electrode so that the charge from each sensor could be measured independently. Unfortunately, this approach would require a large number of wires or traces for connecting all the electrodes to the control electronics, and it would be difficult to route all the wires out to the edges of sensors made up of large, dense arrays.
Therefore, there is a need for an apparatus and method of forming and using a touch sensing device that is configured to solve the problems with piezoelectric sensors described above. The touch sensing device also should be inexpensive to produce, reliable in operation, and be formed so that it can be integrated within a desirably sized electronic system.